Handling hot aqueous solutions of phosphoric acid



Nov. 19,1940. A. B. KINZEL HANDLING HOT AQUEOUS SOLUTIONS O F PHOSPHORICACID Filed 'June' 15, 1959 SILVER-SILICON ALLOY VESSEL HOT PHOSPHORICACID INVENTOR L E w |.Y

m m w m U A Patented Nov. 19, 1940 UNETED S'l' TES HANDLING nor AQUEOUSSOLUTIONS or I rnosrnomc ACID Augustus B. Kinzel, Douglaston, N. Y.,assignor to Electra Metallurgical Company, a corporation of WestVirginia Application June 13, 1939, Serial No. 278,819

I 6 Claims. (01. 23-165) This invention relates to the art of handlinghot aqueous solutions of phosphoric acid and provides a method ofhandling such solutions inapparatus resistant to corrosion by hotphosphoric acid at temperatures between 200 C. and 400 C.

Phosphoric acid has a diversity of commercial uses. For example, itis'used in the manufacture of fertilizers, baking powder, and in themanufacture of phosphates for other purposes. It may also be used as acatalyst in the hydration of olefines. Many difficulties are encounteredin the handling of phosphoric acid due to its corrosive properties.Chromium-nickel steels containing about 18% chromium and 8% nickel havebeen used for handling cold, dilute acid, but are unsatisfactory for usein contact with concentrated acid. In the manufacture of phosphoricacid, lead lined equipment is frequently used, but the use of suchequipment is limited because certain concentrations of phosphoric acidattack lead. When the acid is used inthe manufacture of food productssuch as baking powder, the presence of lead in the acid, resulting" fromits attack on lead equipment, is undesirable since the final product isapt to be contaminated by the lead in the acid. The presence of lead inmore than a specified amount in food products is prohibited byGovernment regulations.

Accordingly, it is desirable that equipment for handling phosphoric acidbe fabricated from a material even more resistant to the corrosiveaction of phosphoric acide than lead. Silver is a material which isresistant to corrosion by phosphates, and steam jacketed silver kettleshave been used in the preparation of sodium salts of phosphoric acid.However, Circular C412 of the Bureau of Standards (published October 2,1936) at page 36 shows that in tests of pure silver in an solution ofphosphoric acid at 210 C. to 231 C. over a period of 16 hours, silverwas attacked at the rate of 35 to 69 mg., cm. /day. At temperatures of112 to 121 C. over a period of 28 hours the corrosion rate in 85%phosphoric acid solution was about 3 mg/cm /day. Thus, while silver isquite resistant to phosphoric acid at temperatures below 200 C. it israther severely attacked at temperatures above about 200 C.

Furthermore, tests which I have conducted have demonstrated that even atmoderately elevated temperatures silver readily softens. In addition, attemperatures above about 200 C. the creep strength of silver isnegligible. Because of these characteristics silver can safely be usedfor the handling of phosphoric acid only at tem temperatures above 200C. It is a further object to provide such apparatus which also retainsits strength at elevated temperatures between about 200 C. and 400C.Another object. is a process for handling hot aqueous solutions ofphosphoric acid which comprises holding such solutions in a containercomposed of a silversilicon alloy. resistant to corrosion by'phosphoricacid and having high creep strength atelevated temperatures. Anotherobject is to provide apparatus which may be used with safety in thehandling of hot phosphoric acid. Other objects will become apparent fromthe following description and the accompanying drawing, in which thesingle figure is a representation of a vessel fabricated from an alloyhaving the desired properties.

The objects of the invention are achieved by the application of mydiscovery that the addition to silver of a eutectic formingorprecipitationhardening elementsuch 'a's silicon produces an alloywhich retains its strength at elevated temperatures and which alsopossesses resistance to corrosion by phosphoric acid at suchtemperatures. This discovery was unexpected and surprising becauseprevious attempts to improve the strength of silver by the addition ofthe usual hardening agentssuch as tin and copper resulted chiefly indetrimentally alfecting its resistance to corrosion by phosphoric acid.

In accordance with the invention, aqueous solutions of phosphoric acid,at temperatures between about 200 and 400 C., are handled in ap paratussuch as, for example, a container, which may consist of a tank, a tubeor other vessel to be used in handling hot phosphoric acid, isfabricated from a silver base alloy containing silicon in a substantialproportion less than about 5%, the silicon being present in the alloy insuch proportion as to impart to the alloy the ability to retain itsstrength at elevated temperatures and to withstand the corrosive attackof phosphoric acid at such elevated temperatures. The silver content ofthe alloy should be at least and ordinarily the siliconcontent shouldnot exceed 2%. Preferably silicon is present in a proportion between 1%and 2%.

In the single figure of the drawing a vessel V is shown for holdingphosphoric acid at temperatures elevated between about 200 C. and

400 C. The vessel V is fabricated from an alloy containing 1% to 2%silicon, the remainder being substantially all silver.

Another embodiment of the invention is a process for handling phosphoricacid at elevated temperatures between about 200 C. and 400 C. whichcomprises holding such acid in a container, or passing it through atube, composed of an alloy of silver and silicon substantially resistantto the corrosive action of phosphoric acid and having high creepstrength at elevated temperatures not exceeding about 400 C., the alloycontaining a substantial proportion of silicon not in excess of 5% butsufficient to impart high creep strength to the alloy.

The invention thus provides apparatus which may be used with safety inthe handling of hot aqueous solutions of phosphoric acid at temperaturesnot exceeding about 400 C. While a specific embodiment of the inventionhas been described and shown in the .accompanying drawing, the inventionis not limited to such specific embodiment but is useful in other formsof apparatus used in handling hot phosphoric acid.

This application is in part a continuation of my co-pending applicationSerial No. 157,696, filed August 6, 1937.

I claim:

1. In the art of handling aqueous solutions of phosphoric acid atelevated temperatures between 200 C. and 400 C., the improvement whichcomprises holding such acid in a container composed of an alloy ofsilver and silicon, said container being substantially resistant to thecorrosive action of phosphoric acid, and having high creep strength atsuch temperatures.

2. In the art of handling aqueous solutions of phosphoric acid atelevated temperatures between about 200 C. and 400 C., the improvementwhich comprises passing such acid through a tube composed of an alloy ofsilver and silicon, said tube being substantially resistant to thecorrosive action of phosphoric acid, and having high creep strength atsuch temperatures.

3. In the art of handling aqueous solutions of phosphoric acid atelevated temperatures between 200 C. and 400 C., the improvement whichcomprises holding such acid in a container composed of an alloy ofsilver and silicon, said container being substantially resistant to thecorrosive action of phosphoric acid, and having high creep strength atsuch temperatures, the silicon being present in the alloy in asubstantial proportion not exceeding 5%.

4. A process for handling phosphoric acid at elevated temperaturesbetween about 200 C. and 400 C. which comprises holding such acid in acontainer composed of an alloy of silver and silicon substantiallyresistant to the corrosive action of phosphoric acid and having highcreep strength at such temperatures, silicon being pres ent in the alloyin a substantial proportion not exceeding 5% but sufiicient to impart tothe alloy high creep strength at elevated temperatures not exceeding 400C.

5. A process for handling phosphoric acid at elevated temperaturesbetween about 200 C. and 400 C. which comprises passing such acidthrough a tube composed of an alloy of silver and silicon substantiallyresistant to the corrosive action of phosphoric acid and having highcreep strength at such temperatures, silicon being present in the alloyin a substantial proportion not exceeding 5% but sufiicient to impart tothe alloy high creep strength at elevated temperatures not exceeding 400C.

6. A process for handling aqueous solutions of phosphoric acid atelevated temperatures between about 200 C. and 400 C. which comprisesholding such solutions in a container composed of an alloy of silver andsilicon substantially resistant to the corrosive action of phosphoricacid and having high creep strength at elevated temperatures notexceeding 400 C., said alloy containing 1% to 2% silicon with theremainder substantially all silver, the silicon imparting to the alloyhigh creep strength at elevated temperatures not exceeding 400 C.

AUGUSTUS B. KINZEL.

